Introduction to Orthodontics
Norman Johnston United Kingdom
Bite defects are common in small general practice. The majority is found in dogs but rabbits and, less often, cats also present with malocclusions. To diagnose bite defects and formulate treatment plans, the veterinarian needs to be able to recognize the normal position. The approach to bite assessment therefore needs to be methodical and logical.
The normal occlusal pattern is termed orthognathism. Problems exist commonly in dogs, mainly due to great variation in skull type within the species.
There are three basic skull types:
Dolicocephalic: long, narrow faces, e.g., Rough Collie, Borzoi or Greyhound.
Mesocephalic: ”normal” type face, e.g., German Shepherd, Siberian Husky.
Brachycephalic: short, broad faces, e.g., Boxer, Bulldog, and Pug.
All adult dogs normally have 42 adult teeth. Clearly the reduction in length of the facial profile in small, short faced breeds will lead to crowding of teeth at the very least, with more painful and disfiguring defects in more extreme cases.
NORMAL BITE ASSESSMENT
The normal points to look for in assessing a dog bite are as follows:
The incisors should be in scissor bite. The upper incisors should overlap the lower incisors. The incisal edges of the lower incisors should occlude at or near the cingulum ridge on the palatal surface of the upper incisors.
The lower canines should fit neatly into the diastema space between the upper canines and corner incisors when the mouth is closed. Ideally, the lower canines should touch neither upper tooth.
The premolars should form “shear mouth” whereby the tips of the crowns of the mandibular premolars should point directly into the interproximal spaces between the premolars on the upper jaw and vice versa. The upper fourth premolar (carnassial) should be lateral/buccal to the lower first molar.
The skull should be symmetrical in the sagittal plane.
Any deviation from the above criteria is a deviation from the normal. Some textbooks advocate systems that include more criteria. Whilst these are useful in more complex cases, the main four criteria will allow a rapid and accurate assessment of the majority of cases seen.
Class 0 or Orthoclusion
This is a normal bite for a dog as per the criteria listed above.
Class One Malocclusion/Neutroclusion
The overall relationship is correct relative to upper and lower jaws but the line of occlusion is incorrect due to one or more teeth being out of alignment, rotated, or changed in some way, e.g., retained deciduous teeth, anterior cross-bite, posterior cross-bite, lingually displaced mandibular canine teeth, missing teeth, impacted teeth, supernumerary teeth, rotated and crowded teeth.
Class Two Malocclusion/Distoclusion/”Overshot”
Some or all of the mandibular teeth are distal in their relationship with the maxillary teeth. In effect, this refers to a short mandible and a relatively long maxilla. Often referred to as an overshot jaw. This is not “normal” in any breed but is seen in a variety of dogs. It is reported to have a proven autosomal recessive mode of inheritance in longhaired Dachshunds and German Shorthaired Pointers. Other breeds certainly display class 2 malocclusions in relatively high numbers also. The Bearded Collie, German Shepherd, West Highland, and Weimeraner are all common breeds in the UK with this problem.
Class Three Malocclusion/Mesioclusion/”Undershot”
Some or all of the mandibular teeth are mesial (rostral) in their relationship with their maxillary counterparts. Refers to a long mandible and a relatively short maxilla. Breeds such as Boston Terriers, Boxers and English Bulldogs show this undershot jaw as a “normal” anatomical feature. These breeds often have overcrowded teeth due to lack of space, particularly in the maxilla, and develop early periodontal disease.
CLINICAL IMPORTANCE OF MALOCCLUSION
The primary responsibility for the veterinarian faced with a malocclusion is the relief of pain or discomfort. The second responsibility is counseling as regards the possible inheritance of the condition. An otherwise sound, well cared for dentition may show asymmetric abrasion or attrition. Problems may occur with mastication and TMJ function. Soft tissue trauma is a common sequel of malpositioned teeth and, finally, premature loss of teeth may be caused by an increased liability to periodontitis.
AETIOLOGY AND ETHICS OF MALOCCLUSION
The primary problem facing the veterinarian as regards aetiology is to decide whether the problem is inherited or acquired. If it is possible to determine this (assisted by the criteria listed above), it will allow the planning of treatment in a rational and ethical manner. Frequently there is more than one treatment available for any one condition and it is critical that animals are not returned to the gene pool with treated inherited orthodontic conditions that make their use as stud dogs more desirable. Many breeds in the UK consistently run with high levels of common malocclusions. Some of these are clinical and clearly visible phenotypically. Some may be marginally important at the clinical level, i.e., cause no functional problems to the animals, but on a genotypic level still be significant to the breed as a whole. These individuals can often be identified using the criteria above and removed from the breeding pool. Failing to take action at this time can lead to levels of significant problems in following generations.
Interceptive orthodontics refers to taking action early, often at the time of temporary dentition, to avoid problems later. The four jaw quadrants all grow independently. In a rapidly growing animal this can commonly lead to temporary malocclusions. The most common is a class 2 malocclusion where the mandibles grow more slowly than the maxillary quadrants leading to a situation where the temporary lower canine teeth become caught distal and lingual to their normal position, possibly in pits in the hard palate tissue. Normal rostral growth of the mandibles is then prevented so the mandibular bodies tend to bow ventrally rather than grow rostrally. Appropriate and careful extraction of the lower temporary canines at this time can relieve the dental interlock and prevent the long term bowing as well as relieving a painful problem. Another example would be the presence of a mixed dentition—temporary and permanent teeth—most often incisors. Clearly extraction of the temporary teeth is desirable to relieve crowding. It is wise to radiograph the area first as inadvertent extraction of a permanent tooth by mistake at this time can result in a very unhappy client.
COMMON CLINICAL MALOCCLUSIONS
This is a defect, which can be inherited or acquired. Possible aetiology’s are retained temporary incisors, crowding of the upper or lower incisor arches or trauma pushing one or more of the upper permanent incisors caudally. Orthodontic manipulation by means of an acrylic brace with an expansion screw or by an archbar with buttons and elastic bands has been reasonably successful in correcting this problem as long as the arch is not crowded. This should not be performed until the skull development is complete otherwise skull growth may outpace the tooth movement. Before embarking on orthodontics of this type bear in mind that the device employed may well have to be present for 12 weeks or more. In addition the veterinarian should carefully examine the ethical position. This is generally not a dysfunctional condition and may be due to crowding of the incisor arch and as such treatment may not be in the best interest of the animal or the breed.
In this condition one side of the mandible and/or maxilla is longer than the other. It leads to an asymmetry in the saggital plane. It is considered to be inherited. Frequently other bite abnormalities will be present simultaneously.
Mandibular carnassial (Molar 1 or 409/309) is located buccal to maxillary carnassial (Premolar 4 or 108/208). Often results in more rapid accumulation of calculus and the area becomes more prone to periodontal disease. Treatments range from diligent homecare to more sophisticated periodontal disease treatments. Odontoplasty may be required if the main cusp causes trauma to soft tissue on the opposite arch. Frequently other bite abnormalities will be present simultaneously.
Base Narrow Canines/Lingually Displaced Mandibular Canines
Probably the most common reason for orthodontic referral due to the pain and obvious soft tissue damage involved. The lower canines erupt in a dorsal direction rather than bucally and fail to find their normal position in the diastema between the upper canine and incisor 3. This usually injures the soft tissue of the opposing hard palate. On its own it would be classified as a Class One malocclusion but in many cases it is combined with mandibular brachygnathism and therefore rates as a mixed Class Two/Class One malocclusion. This condition has been proven to be due to an autosomal recessive mutation in at least one breed (GSH Pointer, ref. Byrne and Byrne. Veterinary Record (1992): 130, 375-376). Treatment is essential in most cases to allow the mouth to close without pain. This can take the form of orthodontic tipping with a bite plane or shortening the lower canine crowns surgically and performing a partial coronal pulpectomy on the exposed pulps. In addition, neutering of the animal should be strongly advised. Extraction of the mandibular canines is to be avoided if possible due to the strength the roots contribute to the symphyseal area.
The key to occlusion or to the type of bite is seen primarily in the relationship of the maxilla to the mandible in the premolar teeth although four separate criteria, at the very least, should be examined for a bite appraisal.
1. The incisors should exhibit scissor bite.
2. The mandibular canine should fit neatly in the diastema between the upper corner incisor and maxillary canine and should touch neither. In addition it should be angled in a lingual buccal direction.
3. The upper PM1 lines up in a slot between the lower PM1 and PM2. The upper PM2 lines up in a slot between the lower PM2 and PM3 and so forth to provide a “pinking shear” effect.
4. The skull should be symmetrical in the sagittal plane
To safeguard the ethical position and to prevent treatments being performed that are not in the best interest of the breed, owners should be asked to sign an orthodontic release form.
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